Analog circuits, whether alone or as part of a mixed-signal integrated circuits (ICs), are particularly susceptible to substrate noise. In mixed signal ICS, the analog circuits are designed on the same semiconductor chip as digital circuits. Consequently, since the analog circuits share the same substrate with high-speed digital circuits, electrical noise created by the high-speed operation is coupled between the circuits and may affect the performance of the analog circuits. As the frequency of the operational digital circuit increases, and transistor dimensions are reduced, the effect of noise coupling is becoming more and more serious for the analog circuits, particularly. Similar problems may also exist for ICs including only analog circuits, as opposed to mixed signal ICs.
The industry has, in an attempt to reduce substrate noise, turned to the use of heavily doped buried layers (e.g., heavily doped buried N-type and P-type layers). Other industries, especially those including specialized High Voltage components (e.g., FET's, SCR's and Diodes as well as BJT components) also use heavily doped buried layers to enable a subsurface region of low resistivity and achieve required performances. For technologies employing such heavily doped buried layers, epitaxial silicon is grown over such buried layers after their formation. However, epitaxial lattice shift, particularly acute when using off-axis silicon substrates, often results in misalignment with the heavily doped buried layers. The result of the unknown epitaxial lattice shift, at least in one instance, is a misalignment between the heavily doped buried layers edges and the well edges formed thereover. This is particularly problematic for high voltage wells, which may lead to significant leakage paths or changes in the high voltage component capability. No known method for measuring epitaxial lattice shift in a non-destructive manner on product wafers currently exists.
An urgent need has, therefore, arisen for a method for accurately measuring epitaxial silicon shift, such that the aforementioned misalignments between sub-surface features (e.g., heavily doped buried layers in one instance) and surface features (e.g., well structures in one instance) may be reduced.